epoll: eliminate unnecessary lock for zero timeout

We call ep_events_available() under lock when timeout is 0, and then call
it without locks in the loop for the other cases.

Instead, call ep_events_available() without lock for all cases.  For
non-zero timeouts, we will recheck after adding the thread to the wait
queue.  For zero timeout cases, by definition, user is opportunistically
polling and will have to call epoll_wait again in the future.

Note that this lock was kept in c5a282e963 because the whole loop was
historically under lock.

This patch results in a 1% CPU/RPC reduction in RPC benchmarks.

Link: https://lkml.kernel.org/r/20201106231635.3528496-9-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Cc: Guantao Liu <guantaol@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1743,7 +1743,7 @@ static inline struct timespec64 ep_set_mstimeout(long ms)
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		   int maxevents, long timeout)
 {
-	int res, eavail = 0, timed_out = 0;
+	int res, eavail, timed_out = 0;
 	u64 slack = 0;
 	wait_queue_entry_t wait;
 	ktime_t expires, *to = NULL;
@@ -1759,18 +1759,21 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 	} else if (timeout == 0) {
 		/*
 		 * Avoid the unnecessary trip to the wait queue loop, if the
-		 * caller specified a non blocking operation. We still need
-		 * lock because we could race and not see an epi being added
-		 * to the ready list while in irq callback. Thus incorrectly
-		 * returning 0 back to userspace.
+		 * caller specified a non blocking operation.
 		 */
 		timed_out = 1;
-
-		write_lock_irq(&ep->lock);
-		eavail = ep_events_available(ep);
-		write_unlock_irq(&ep->lock);
 	}
 
+	/*
+	 * This call is racy: We may or may not see events that are being added
+	 * to the ready list under the lock (e.g., in IRQ callbacks). For, cases
+	 * with a non-zero timeout, this thread will check the ready list under
+	 * lock and will added to the wait queue.  For, cases with a zero
+	 * timeout, the user by definition should not care and will have to
+	 * recheck again.
+	 */
+	eavail = ep_events_available(ep);
+
 	while (1) {
 		if (eavail) {
 			/*
@@ -1786,10 +1789,6 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		if (timed_out)
 			return 0;
 
-		eavail = ep_events_available(ep);
-		if (eavail)
-			continue;
-
 		eavail = ep_busy_loop(ep, timed_out);
 		if (eavail)
 			continue;